View full sizeA mauve stinger drifts in the open Gulf on Saturday, about ten miles off the Alabama coast. The jellies are infrequently seen in the Gulf and pack a painful sting. (Ben Raines/al.com)

Tens of thousands of small jellyfish, their bells covered in violet dots, pulsed through clear blue water off Alabama last week, long trails of tentacles wafting behind them.

Known as mauve stingers, the jellyfish are infrequent visitors to this part of the Gulf of Mexico, though they turned up in large numbers last year as well. Scientists say the species typically shows up in an area two or three years in a row and then disappears for a decade or more.

Their Latin name, Pelagia noctiluca, translates as sea night light, a reference to their ability to glow in the dark when disturbed.

Monty Graham, head of marine sciences at the University of Southern Mississippi, said the jellies showed up off of Alabama in 1998 and were seen for about two years before disappearing until 2011.

"We're definitely in a jellyfish cycle. It tends to be on these decadal cycles," Graham said. He cited a doctoral thesis by Kelly Robinson at the Dauphin Island Sea Lab that highlighted the roughly 10-year cycle between years with lots of jellyfish in the Gulf.

"These guys, their life cycle is different than our other coastal species. These don't have an attached polyp stage, so they don't need to be close to shore. These tend to be further offshore, more of an oceanic species," Graham said. "The same species causes a lot of problems in the Mediterranean."

View full sizeDozens of mauve stingers can be seen floating in this image. The jellies have a painful sting and have turned up off the Alabama coast this fall. (Ben Raines/al.com)

They are noted for having a particularly painful sting that can leave purple marks, which linger for several days.

Last year, several thousand swimmers were stung in Cocoa Beach during the Memorial Day weekend. The invasion made national news. Similar invasions have been documented in Italy.

The animals appear to be present in large numbers off Alabama. During a scuba dive 10 miles south of Fort Morgan last week, jellies filled the water column beginning about 10 feet below the surface and stretching down to a depth of 45 feet. They were spaced about a foot apart. Thousands were visible in every direction, each about the size of a plum.

The spectacle of so many transparent, violet-hued creatures gently pulsing and drifting against an aquamarine backdrop was mesmerizing and lovely to behold. The unavoidable stinging tentacles provided a less-pleasing sensation.

The animals have been in the area much of the summer, though their numbers have increased steadily. Sections of the beaches of Dauphin Island and Petit Bois were covered in a gelatinous layer composed of washed up mauve stingers after Hurricane Isaac's passing.

Mauve stingers covered portions of Alabama beaches in a thick, gooey layer the day after Hurricane Isaac passed. (Ben Raines/al.com)

The jellyfish are carnivorous, eating small fish, minute marine organisms such as copepods, and fish eggs and larvae.